In recent years, there have been increasing expectations for robots operable in areas close to humans, such as medical robots, domestic service robots, and operation support robots in factories. For these robots, it is important to ensure safety in the event of occurrence of contact with a person, unlike industrial robots. This requires realization of all of power enabling operations and reduced weight and inertial and, further, requires joint driving actuators having high power-to-weight ratios. As examples of actuators which meet these requirements, there have been oil hydraulic actuators (refer to Non-patent Document 1, Patent Document 1, and Patent Document 2, for example). Oil hydraulic actuators are capable of driving at high pressures using a non-compressible fluid, which enables provision of higher power-to-weight ratios.